The movement of work product through different assembly stations or work cells has led to dramatic improvements in production yield and efficiency. Some robot-assisted assembly lines convey a product to the different work cells via an overhead or an underbody carrier, and then stop the product at a particular cell. Robots may perform assigned work tasks before the line is restarted. In other assembly line configurations, the robot may track assembly line motion and perform automated tasks on the moving product.
The synchronized motion of a moving assembly line presents unique control challenges. For instance, the robot's motion must be sufficiently coordinated with the motion of the assembly line, e.g., a particular carrier which transports the work piece to the different stations or cells in the plant. Once proper alignment is achieved between the robot and the work piece, and once the robotic assembly task commences, the robot and to-be-assembled work piece are in mutual contact. In this contact phase of control, the robot's motion is controlled as a function of the contact forces between the end of arm/end effector of the robot and the product.
Moving robotic assembly lines are also equipped with a host of safety devices such as pressure mats, gates, light curtains, manual stop buttons, and other emergency stop (e-stop) devices. Typically, triggering of an e-stop event immediately stops the robot. However, if an e-stop event occurs during the contact phase of control, the disruption of synchronized motion between the robot and the product may be abrupt. In some instances this disruption can lead to a collision between end effector and product, and as a result, an undesirable, albeit transient, impact force on the product and robot end effector.